The field of the invention is the implantation of permanent magnets in bone to reduce the contact pressure between bones at a joint or between a bone and a prosthetic device.
Primary osteoarthritis is a disabling condition, which destroys the joint surfaces through degenerative changes, as age advances. It is the leading cause of limitations in activities of daily living and is second only to heart disease in causing work disability. The direct traditional medical costs and indirect economic and wage loss from arthritis in individuals in the United States has reached an excess of $65 billion annually. In 1994, the Center for Disease Control, reported that by the year 2020, osteoarthritis will have the largest increase in the numbers of new patients of any disease in the United States.
Initial management of most arthritis patients includes changes in lifestyle, NSAIDs, analgesics, physical therapy, bracing and ambulatory aids. Surgical treatment comes into play only when consecutive treatment fails to improve the symptoms. Common surgical options include arthroscopic debridement of the knee, high tibial osteotomy, and unicompartmental or tricompartmental knee replacement, depending on the predominantly involved compartment. Surgical procedures, short of joint replacement surgery or high tibial osteotomy which include arthroscopic lavage, microfracture, chondrocyte or osteochondral transplants are not cost effective and have no reliable long term results.
High tibial osteotomy is mostly done in patients who have osteoarthritis and have varus malaligned knees. It relieves pressure from the medial joint line and redistributes the body weight passing through the knee so that the lateral compartment, which is relatively spared from the wear and tear process of osteoarthritis, can bear most of the weight. Pain is relieved and function is restored in more than 60% of the patients even 10 years after the operation. The results are best if at least 7 to 10 degrees of valgus alignment is achieved at operation and the weight of the person is not more than 30% of the ideal body weight. Clearly, procedures which reduce the pressure between arthritic bones reduces pain.
The idea of implanted magnets to produce forces between bones at a joint or between a bone and a prosthesis has been known for many years. The use of permanent magnets in bones to either assist in holding joints together or to reduce pressure between two bones at a joint or a bone and a prosthesis was disclosed initially in U.S. Pat. No. 4,024,588. More recently the use of arrays of permanent magnets implanted in bone have been disclosed in U.S. Pat. Nos. 5,507,835; 5,879,386; and 6,387,096 and published U.S. Appln. 2002/0032484A1. The permanent magnets are either mounted in prosthetic elements which are fastened to a bone, or the permanent magnets are encased in a biocompatible material and inserted into a hole drilled in the bone.
There are two practical problems with prior magnet implantation methods which have precluded their clinical use. First, there is no method suitable for clinical use to determine the strength of permanent magnets to be implanted and there is no suitable method for implanting the permanent magnets in bone.